Fluoride phosphors



E. L. LIND 2,757,144

July 3l, 1956 FLUORIDE PHosPHoRs Filed Feb. 24, 1954 @a @15e/:2: 0.0M MN 1N V EN TOR. bw/m0 l /A'D 2,757,144 Patented July 31, 1956 tice FLUORIDE PHOSPHORS Edward L. Lind, Princeton, N. J., assignor to Radio Corp poration of America, a corporation of Delaware This invention relates to luminescent materials, also knownl as phosphors, and particularly to improved luminescent materials of the iluoride type and to improved methods for preparing these materials and luminescent screens including said materials. t

it is often desirable to provide a blue-emitting luminescent material exhibiting'a long, exponential decay of` phophorescence emission upon removal of cathode ray excitation. Heretofore, blue-emitting luminescent materials have either had a short exponential decay or a long power-law decay. The advantagev of amaterial having an exponential decay is that, asl the temperature of the material rises and as the excitation intensity increases, the rate of change of phosphorescence with respect to time remains substantially con-stant.- The temperature rise may be due, for example, to continued cathode ray excitation or to increased cathode beam current. ln addition a material having an exponential decay provides a more uniform phosphorescence emission during the periods between successive excitations as compared to other knownl types of decay.

Luminescent materials having a long decay are frequently used in cathode ray tubes for radar purposes, where it is desirable for the emission to remain upon the luminescent screen for a relatively long time.

Luminescent materials, having a somewhat shorter decay than that used for radar purposes, may be used in luminescent screens of kinescopes of television receivers. Where the television receiver is located many miles from the transmitter, in the so-called fringe areas, there is a substantial amount of noise with the signal which appears as random white spots, or snow, upon the luminescent screen of the kinescope. This effect may be minimized to the viewer by using luminescent materials in the screen of the kinescope having a longer decay time. By using vlong-decay materials, the luminescent image on the kinescope screen dies away more slowly, allowing the image areas of the luminescent screen to build up to a stronger intensity, while allowing the random noise signals to die away in the background areas. A white-emitting kinescope screen often comprises a blueemitting luminescent material and a luminescent material emitting light of a complementary color. Blueemitting luminescent materials may also be used in tricolor kinescopes as one of the component colors.

lt is an object of this invention to provide improved fluoride-type luminescent materials. t

A further object is to provide blue-emitting luminescent materials having a long, exponential decay.

Another object is to provide improved methods and means for preparing fluoride-type luminescent materials.

ln general the invention includes luminescent materials comprising manganese-activated calcium beryllium lluorides. The invention also includes manganese-activated calcium beryllium iluorides wherein up to about 25 mole per cent of said calcium has been replaced with another element selectedy from group Il ofthe periodic table. The invention includes methods of preparing these luminescent materials comprising reacting components yielding uorine, calcium, beryllium and activator quantities of manganese. Up to 25 mole per cent of the calcium may be replaced with another element selected from group ll of the periodic table. According to a first embodiment, the components are permitted to react in an excess of hydrofluoric acid, subsequently drying and, optionally, tiring the reacted mixture. According to a second embodiment, the materials may be red without reaction in hydrouoric acid.

The foregoing objects and other advantages will be more apparent and the invention will be more completely described by reference to the accompanying drawing in which:

Figure 1 is a spectral distribution curve of the emission of CaF2-BeF20.006Mn under cathode ray excitation;

Figure 2 is a set of curves illustrating the phosphorescence decay characteristics of CaF2-BeF2:O.006Mn compared to 2ZnOSiO2:0.006Mn; and t Figure 3 is a sectional view of a cathode ray tube bulb during the preparation of a luminescent screen on the face plate thereof.

An example of preparing a preferred manganese activated calcium uoride is as follows: A mixture is prepared of the following ingredients:

Grams Calcium carbonate 10.0` Beryllium oxide 2.5 Manganese (as manganese fluoride) 0.033

These ingredients are preferably of the highest degree of purity obtainable. This mixture is added to a large excess over stoichiometric proportions of a 50 per cent aqueous hydrofluoric acid solution in a platinum crucible or other acid resistant container. The hydrouoric acid is present in a large excess in order to minimize the hydrolysis of the materials that are present. The ingredients are permitted to react and the resulting product is dried rapidly at about to 150 C. The dried residue is moistened with aqueous hydroiluoric acid, in order to recouvert any hydrolyzed material back to the unhydrolyzed form, and then redried slowly without boiling. The redried residue is transferred to a carbon Crucible, covered to minimize oxidation of the residue and then tired at about 750 C. for about l hour. rhe red residue is cooled, ground to a desired degree of neness, replaced in a covered carbon Crucible and retired for about 30 minutes at about 700 C. The retired residue is cooled and ready for use as a luminescent material.

The above described luminescent material is a gritty, white to light gray powder having the approximate formula CaF2-BeFaz0-006Mn- Referring now to Figure 1, the powder is a blue-emitter when excited by cathode rays, having its peak emission intensity at about 4750 A. The ICI coordinates of this material are =.l04, =.140. The crystal structure is tetragonal with the zircon structure.

The time constant, that is the time required for the phosphorescence emission to fall to l/e of its maximum value when excitation is removed, is about 70 milliseconds. Referring to Figure 2, the decay, or decrease in phosphorescence emission, is substantially exponential at the outset of the decay followed by a long power law decay.

The compositions of the above-described luminescent materials may be varied as follows: Up to about 25 mole per cent of the calcium present may be replaced with another element selected from group II of the periodic table such as magnesium, zinc, strontium, cadmium, barium, mercury and radium. In some cases a very marked improvement' is obtained by such a substitution. For example', when' 0.05* mole per centy `of' the calcium present is replaced with cadmium, the time constant of the luminescent material increases up to 30% and the luminescence efficiency increases up to about` 30%.

rPhe moleratio of beryllium to other elements ofI group II of theV periodic table may be varied from :01 to 9. Where` the'ratio' is less than 1, theproduct' is believed to be a mixture of manganese activated calcium fluoride and manganese activated` calcium beryllium iluoride. However, the' preferred ratiov is 1.

The manganese may be present in concentrations between 0`;0006` to 0.12 mole of manganese per mole of calcium and other group II elements which may replace it. The preferred concentration isv 0.006 mole. As` the concentration of manganese increases the colorl of the fired powder changes from white to almost black.

The elements selected from group Il' of the periodic table may be introduced in any convenient form. For example, to introduce calcium; calcium oxide, calcium hydroxide, calcium bicarbonate, calcium carbonate or calcium fluoride may be used. Calcium is preferably' introduced as the carbonate, however, and berylliumas theV oxide.

In the example, when the mixture is completely dissolved in hydrouoric acid and dried, itis luminescent. However, the luminescent properties may be improved by firing at an elevated temperature. Firing is preferably accomplished in a covered carbon'v Crucible or inv areducing atmosphere, such as hydrogen, hydrogen fluoride or carbon monoxide. The material may also be fired inan inert gas such as nitrogen. The material may beI red for any length of time and the firing should be carried out below the temperature at which there is a liquid` phase present. This temperature will depend upon the composition. It is preferred to re the material of thev example for about 1 hour aty about 700 C.

The following table gives the observed time constant of materials having the composition CaF2'BeF2:0.006Mn red for 30 minutes at various temperatures.

Temperature: Time constant, milliseconds Untired 57 400 C 64 500" C 59 600 C 64- 700 C 69 800 C 71- The step of grinding and retiring the tired luminescent material is optional. It has been found convenient to rere in order to obtain a fine particle size in the iinal'. product.

An alternate method of preparing the luminescent materials of this invention is to ire a mixture of nely' powdered components at the desired temperature and for the desired period of time. For example, one mole p'art of powdered calcium fluoride, one mole part of beryllium uoride and 0.006 mole part of manganese fluoride may be mixed and then fired in a graphite crucible at about 750 C. for about one hour. The foregoing description applies equally to this alternate method of preparation.

The luminescent compositions of the invention have the following general formula: aCaFrbMFrcBcFz'zdh/in wherein a may have values between. 0.75l and 1.00, a -I-b 1, c may have values between 0.01 and 9, d may have values between 0.0006 and 0.12' and M is an element selected from the group consisting of magnesium, zinc, strontium, cadmium, barium', mercury and radium. The preferred compositions are CaFz-BeF2z0-006Mn and 0;95CaF2'0.05CdF2BeF2:0.006Mn.

Referring to Figure 3, luminescent screens may be prepared using the luminescent materials of the invention. For example, a convenient method of preparing a luminescent screenl for a cathode ray tube is to prepare a about 350 C. for about 1/2 hour.

suspension of the luminescent material in a liquid mediurn, which medium may be aqueous or non-aqueous such as acetone or methanol. The suspension of the luminescent material is poured through the neck 25 into a cathode ray tube bulb resting on its face plate 24. The suspension fills the bulb up to a convenient level (L-L) of the conical part 26 of the bulb. Screen 28 is formed on face plate 24 by settling from* the suspension 29. A conductive coating 27S may be appliedito'a' part of neck 25 and of conical part 26 of the bulb. After the luminescent screen 2-8l has been settled, baking may be carried out at about 350' C. for about 1/2 hour with a flow of dried air through the' bulbl followedy by an exhaust bake at For example, the face plate 24 may b'ecoated with a tacky material and the dry luminescent powder material dusted on. Or the powdered luminescent material may be mixed with a tacky material and the mixture coated on the face plate.

There have been' described improved fluoride-type luminescent materials, and methods and means for preparing them', and for preparing cathode ray tube screens employing such phosphors. These materials exhibit a blue emission upon cathoderay excitation and a long exponential deca-y upon the removal of the cathode ray excitation. These materials are useful in cathode ray tubes for radar purposes, and as a blue-emitting component in the luminescent screens of kinescopes for use infringe areas and as the blue-emitting component in tricolor kinescopes.-

What is claimed' is:.

1. A luminescent material comprising a manganeseactivated calcium beryllium fluoride wherein 0.0 to about 25:0 mole per cent of said calcium hasbeen replaced with another element selected from group 1I of the periodic table except beryllium the mole ratio of beryllium to other elements of group I'I of the periodic table being in the range between 0.0'1 to 9.0.

2. A luminescent material having the molar composition aCaF2bMF2cBeFz1dMn where a\-b'=1, a is 0.75-l.00, c' is 0.01-9, d is 0.0006-0.12 and M is an element selected from the group consisting of magnesium, zinc, strontium, cadmium, barium, mercury, and radium.

3. A luminescent material. comprising a calcium beryllium fluoride having about 0.0006 to about 0.12 mole of manganese activator per moleV of calciumv and cadmium, wherein cadmium has replaced calcium in an amount up to. 25 mole: per cent of the calcium the mole ratio of beryllium to calcium plus cadmium being in the range between 0.01 to 9.0..

4- A luminescent material comprising a calcium beryllium fluoride having about 0.0006 toV about 0.12 mole of manganese activator per mole of calcium the mole ratio of beryllium to calcium being about 1.

5. A luminescent material having the molar composition CaFzBeF2:0.006Mn.

6. A luminescent material having the molar composition 0.95CaF2'0.05CdFz-BeFzr0006Mn..

7. A luminescent screen comprising a base and a coating thereon including a luminescent material having the molar composition aCaF2bMF2cBeF2:dl\/ln where a-l-b=l, ais 0.75-1.00, cis 0.01-9, d is 0.0006-0.12 and My isan element selected from. the group consisting of magnesium, zinc, strontium, cadmium, barium, mercury, and radium.

8. A luminescent screen comprising: a base and a coating thereon including a luminescent material having the molar composition 0.95CaF2-0'.O5CdF2BeF2:0.006Mn.

9. A method` of preparing. a luminescent material comprising tiring at about 750 C. for about one hour a mixture of ingredientsin the following proportions: 1.0 mole of calcium as the fluoride, about 0.01. to 9.0 mole of beryllium as the fluoride and 0.0006 to about .12A mole of manganese activator as the fluoride.

10. A method according toclaim 9`f wherein up to 25 mole per cent of the calcium has been replaced with cadmium as the uoride and said manganese is present in proportions of 0.0006 to 0.12 mole per mole of calcium and cadmium.

11. A method according to claim 10 wherein 0.05 mole per cent of calcium has been replaced with cadmium.

12. A method according to claim 10 wherein said mixture is red in a reducing atmosphere.

J. Electrochem. Soc. vo 189-194.

1. 101, No. 4, April 1954, pp. 

2. A LUMINESCENT MATERIAL HAVING THE MOLAR COMPOSITION ACAF2-BMF2''CBD''2; DMN WHERE A+B=1, A IS 0.75-1.00, C IS 0.01-9, D IS 0.0006-0.12 AND M IS AN ELEMENT SELECTED FROM THE GROUP CONSISTING OF MAGNESIUM, ZINC, STRONTIUM, CADMIUM, BARIUM, MERCURY, AND RADIUM. 